What exactly is Pyrolysis – why do I care and how does it apply to wood burning systems?
Pyrolysis is a breakdown of organic material which occurs at high temperatures in the absence of oxygen. It is an irreversible physical and chemical change in the organic material and can be extremely dangerous. The word is from the Greek words pyro and lysis (meaning fire and separating). In the world of wood burning appliances we observe most often when flue gas carrying systems (pipes and chimneys – often masonry chimneys) are too close to combustible material (often wooden structural material). The process occurs around 200°C to 300°C and essentially is a charring of the wood. It is very dangerous and the only reason the wood did not ignite was due to insufficient oxygen present. Two problems really are present when pyrolysis has occurred – first the ignition point of the wood will actually be lower than it was before the process took place – second if oxygen concentration does reach a high enough level at that point in the future combustion will almost certainly occur.
Many Chimney Sweeps and/or Installers would have seen this – most often in older buildings. Unfortunately many WETT Certified SITE Basic Inspectors neither have seen nor have necessary experience to pick up clues when it does exist.
Fact is that normally wood will ignite no lower than around 200°C, but wood that has been seriously impacted by pyrolysis can ignite at temperatures as low as 100°C! Problem is that wood which has been impacted by pyrolysis is generally not visible, if you recall one reason this occurs is low oxygen levels, hence it is often buried deep in wall and close to hot combustion gas carrying pipes/chimneys. Sometimes pyrolysis can slowly progress, even over several decades, but this does not make it any less dangerous.
Keep in mind that any improperly installed fireplaces or wood stoves can cause pyrolysis of the wood in the floor or walls around the appliance.
Pyrolysis can be caused by any wood stove, masonry fireplace, factory built gas fireplace, or any appliance which during operation produces temperatures of over 100°C for extended periods. It is critically important to have a Level 2 Inspection when you purchase ANY property with wood burning system – a further inspection should be performed with each Chimney Sweep annually – be sure to hire ONLY Certified Chimney Sweeps!